Communication systems require drivers to transmit signals across communication media. Often the output signal of the driver is required to be a set amplitude as the communication media is lossy and will attenuate the transmitted signal. The signal at the far-end of the communication media must be large enough to be interpreted by the receiver with no error.
Many communication standards for high-speed serial link components require absolute minimum and maximum output amplitudes. Standards addressing high speeds are often stringent, requiring relatively accurate driver output amplitude. Furthermore, output amplitude is usually addressed after connectors and packaging, which impose frequency-dependent signal attenuation. Package and connector loss can easily approach 20%-30% at high data rates. However, the loss is variable due to various package types or chip placement in a given package. Consequently, one driver output amplitude will not necessarily meet the requirements across various packages and connectors.
Since package and connector loss can be significant at high data rates, ISI (intersymbol interference) is introduced. ISI is the phenomenon where relatively low-frequency data is superimposed onto high-frequency data such that the data transmitted at the different data rates are not well-defined pulses at the system output. Data recovery becomes more difficult and error-prone when ISI is present in the system. Timing jitter is also introduced by ISI.
Many drivers utilize FIR filters to implement signal equalization to combat ISI. Equalization is an approach where the low-frequency energy components are decreased relative to the high-frequency energy components of the transmitted data in a bandlimited system. Low-frequency data is attenuated relative to the signaling Nyquist frequency, thus flattening the overall system response and removing ISI. The most common approach to driver equalization is pre-emphasis, where the high-frequency data amplitude is increased whenever a bit transition occurs. Pre-emphasis is implemented with an FIR filter such that weighted past data is subtracted from the present transmitted data.
Communication systems require a set amount of equalization to ensure ISI is limited at the receiver. Also, some standards require a minimum and maximum amount of transmitter equalization prior to the communication media. However, achieving accurate equalization for all possible packages is difficult due to variable package and connector loss.
Prior art driver/FIR systems and methods may set output levels based on biasing alone. However, biasing solutions have limited performance possibilities. Required nominal value outputs set by biasing alone may not meet minimum values required due to signal attenuation. Moreover, biasing errors within a range of plus or minus 1 to 20 percent are common. The combination of the bias tolerance and signal attenuation results in a composite error violating specifications.
Package and connector loss is detrimental in high speed serial link applications given the tight voltage swing and equalization limits required. What is needed is a method and system for performing effective equalization at very high performance with very low cost in area and power in high speed serial link applications and, in particular, in the backplane link environment.